Thermal desorption methods to remove volatile contaminates from soils in situ are suggested in, for example, U.S. Pat. Nos. 4,973,811, 5,076,727, 5,152,341, 5,190,405, 5,193,934, 5,221,827, and 5,271,693. Methods of applying heat include microwave and radio frequency electrical power along with resistance heating between electrodes; injection of hot gases, and conduction of electricity through the soil. Conductive heat transfer from heat injection wells are suggested in, for example, U.S. Pat. Nos. 5,190,405 and 5,271,693. U.S. Pat. No. 5,271,693 suggests a heater well through which vapors are extracted from the formation.
These methods generally rely on maintenance of a low pressure at the surface or at a vapor extraction wellbore to control movement of contaminates from their initial position to a point where they can be recovered. Because of the relatively large pressure differential needed to cause flow of a significant flux of vapors through soil, only low heating rates can be applied by these methods or pressure will be created at the point where vaporized contaminates exist sufficient to cause the vaporized contaminates to move away from the initially contaminated soil in directions other than the direction in which removal of contaminates is provided for. It is therefore an object of the present invention to provide a method to remove contaminates from a contaminated soil by in situ heating wherein the vaporized contaminants do not move downward from the contaminated soil.